I would like to see something that can not be found in Google))
PU-21 LMG ("Poplin" project) and AEK 906 9х18mm (AEK 906-01 9x19mm) very interesting, but long-known models of weapons.
The photos you submitted earlier (curved barrel RPK and experimental underwater machine gun) are much more interesting.
The photos of PU-1, PU-2 provided by Belezarius, and a photo of the experimental cartridges for the anti-tank rifle PEC provided by Khand-e, also attracted my attention.